Silica-alumina compositions have been tested or studied for various properties to determine the characteristics of such materials in several different contexts. One such study examined the properties of hydroxyl groups on mixed silica-alumina gels involving from 0% to 50% alumina by adsorbing weak H bond accepting molecules onto the gels and recording the OH stretching infrared band. This study revealed the unexpected result that superficial OH groups were not found to be more acidic than those on the silica gel although adsorbed ammonia or pyridine molecules are protonated by the surface. Rouxhet and Sempels,"Hydrogen Bond Strengths and Acidities of Hydroxyl Groups on Silica-Alumina Surfaces and in Molecules in Solution," Journal Chem. Soc. Faraday Transactions, 70, 2021-2032, [1974].
Another study reported by Fripiat, Leonard, and Utterhoeven in The Journal of Physical Chemistry, Vol. 69, No. 10, pp. 3274-3279, Oct., 1965, examined surface acid sites for silica-aluminas with compositions ranging from pure silica to pure alumina. Specifically, schematic structures derived previously from coordination number measurements and infrared spectroscopy had suggested different possible origins for Bronsted and Lewis acid sites according to both the hydration state and surface structure, thereby reflecting to some extent the organization in the bulk. One aspect of this study was the observance of a linear relationship in the variation of surface density in NH.sub.3 irreversibly adsorbed by silica-alumina samples with varying proportions of alumina.
A study by Holm, Bailey, and Clark entitled "Acidity Studies of Silica-Alumina Catalysts" reported in the Journal of Physical Chemistry, Vol. 63, No. 2, pp 129-133, Feb. 1959, examined the relationship between acid content and catalytic properties for various compositions of silica-alumina. The results indicated a non-linear relationship in these aspects: The protonic acid content peaked where the composition was approximately 80% by weight of silica; the polymerization of propylene and hydrogen transfer peaked in the neighborhood of about 70% silica by weight.
Consequently, there is some basis in the prior art indicating a degree of non-linearity in certain observed characteristics of silica-alumina compositions as the ratio of silica to alumina is varied.